In the field of switches, the term "pretravel" refers to the extent of downward movement of a plunger prior to the point at which an electrical circuit is transferred. The point at which the electrical circuit is transferred during the actuating movement of the plunger is the "operating point". The "release point" is the point at which the circuit is transferred back to its original state during return movement of the plunger. The distance between the operating point and the release point is termed the "movement differential". "Overtravel" is the distance that the plunger may move after the circuit is transferred at the operating or release point.
Snap action switches of the type used as limit switches typically have a characteriestic of operation as shown in FIG. 1. In FIG. 1 a plot is made of the force versus travel of the plunger or other actuator which is used to activate the snap action switch mechanism. Such snap action switches exhibit fixed but separate operating and release points, while butting contact switches exhibit the same fixed operating and release point.
As illustrated in FIG. 1, a plunger of a prior art limit switch first must move a pretravel distance 10 before reaching an operating point 16 of the switch. The switch may continue to receive actuating force so that its plunger moves into the overtravel region of the curve indicated as 14. The overtravel is the distance the plunger may move if it continues to receive an increasing actuating force after the operating point 16 is reached. Upon release, as the plunger begins to return to the original position to revise the switch condition, the release point 12 is reached. Movement differential is the difference between operating point 16 and release point 12.
In the prior art switches, the switch design inherently requires that actuating plunger travel must be the same as the return plunger travel (movement differential plus overtravel) before the switch will transfer the circuit to the original position. This means that a fixed operating point and a fixed release point, as the points 16 and 12, are determined for a given switch and do not vary. It further means that the actuation device or the switch mounting or both must be designed to allow for manufacturing tolerances in the involved apparatus components and still provide sufficient movement so that plunger travel through the operating point into the overtravel region is present to ensure that the switch will change states each time it is actuated. In many instances the attainment of these relations is complicated by the fact that normal manufacturing tolerances alone can be greater than the total travel (movement differential plus maximum permitted overtravel) of the switch thereby requiring the use of expensive and complicated mounting bracketry.
The present invention is directed to overcoming one or more of the above problems.